Many processes and catalysts are known for the preparation of homopolymeric or copolymeric olefins and other polymers. Ziegler-Natta catalyst compositions, developed in the 1950s, were found to be particularly useful in the preparation of polyolefins. These catalyst compositions comprise transition metal compounds such as titanium tetrachloride and an alkylaluminum (e.g., triethylaluminum) cocatalyst. The systems were found to be advantageous because of their high activity, and were largely consumed during polymerization.
More recent catalyst systems for use in preparing polyolefins and other polymers are "metallocenes." The term "metallocene" was initially coined in the early 1950s to refer to dicyclopentadienyliron, or "ferrocene," a structure in which an iron atom is contained between and associated with two parallel cyclopentadienyl groups. The term is now used to refer generally to organometallic complexes in which a metal atom (not necessarily iron) is coordinated to at least one cyclopentadienyl ring ligand.
In contrast to the traditional Ziegler-Natta catalysts, metallocenes can provide a polymer composition containing a plurality of polymer molecules of substantially the same molecular structure. That is, if one high purity metallocene catalyst is used, the variance in the composition or molecular weight of the individual polymer molecules produced is minimal. With metallocenes, then, it is possible to control compositional distribution and other aspects of polymer molecular structure with unprecedented precision. Metallocene catalysts have other advantages as well. For example, metallocenes: (a) can polymerize almost any vinyl monomer irrespective of molecular weight or steric considerations; (b) provide the ability to control vinyl unsaturation in the polymers produced; (c) enable polymerization of .alpha.-olefins with very high stereoregularity to give isotactic or syndiotactic polymers; and (d) can function as hydrogenation catalysts for polymers as well as monomers. A. D. Horton, "Metallocene Catalysis: Polymers by Design," Trends Polym. Sci. 2(5):158-166 (1994), provides an overview of metallocene catalysts and their advantages, and focuses on now-conventional complexes of Group IV transition metal complexes and cyclopentadienyl ligands (Cp.sub.2 MX.sub.2, wherein Cp represents a cyclopentadienyl ligand, M is Zr, Hf or Ti, and X is Cl or CH.sub.3).
Horton, supra, discusses the utility of "uniform site" metallocene catalysts relative to traditional "multi-site" Ziegler-Natta polymerization catalysts, and emphasizes the ability to control polymer structure and properties by simply varying the catalyst structure. The catalysts proposed by Horton are homogeneous in nature, in contrast to the typically heterogeneous catalysts used in the preparation of polyolefins.
In addition, certain types of metallocene catalysts have been used to produce polymer compositions that are bimodal (or "bimolecular") or multimodal (or "multimolecular"). A composition referred to as "bimodal" or "multimodal" is generally, although not necessarily, bimodal or multimodal with respect to molecular weight distribution, i.e., the composition has two or more different molecular weight distributions, as may be determined, for example, by the appearance of two or more peaks in a gel permeation chromatogram. However, bimodality or multimodality can refer to other characteristics of a polymer composition as well, e.g., compositional distribution (the distribution of comonomers within a copolymer), tacticity distribution (wherein a polymer contains at least two segments of differing tacticity, long-chain branching distribution, or the like. Polymeric compositions that are multimodal are frequently more useful than compositions that are not; for example, multimodal polymer compositions can have improved rheological behavior, higher mechanical strength and increased elasticity relative to corresponding compositions which are not multimodal.
U.S. Pat. No. 5,525,678 to Mink et al. provides a supported catalyst composition for producing a polyolefin resin having a high molecular weight component and a low molecular weight component, wherein the catalyst composition contains a first catalyst which is a metallocene and a second catalyst which is a non-metallocene. The ratio of the high molecular weight and low molecular weight components in the polymeric product is determined by the ratio of the concentration of the two metals in the two-component catalyst composition. In addition, U.S. Pat. No. 4,659,685 to Coleman, III et al. pertains to a two-component catalyst composition for preparing polyolefins having a molecular weight distribution which is multimodal, the catalyst composition comprising a mixture of a supported titanium compound and a separately supported or non-supported organometallic compound.
U.S. Pat. No. 5,032,562 to Lo et al. also relates to a supported olefin polymerization catalyst composition for producing high density polyethylene ("HDPE") having a multimodal molecular weight distribution. The catalyst composition comprises: (1) a catalyst precursor supported on a porous carrier, and (2) a catalyst activator in the form of a mixture of conventional Ziegler-Natta cocatalysts. Katayama et al., "The Effect of Aluminium Compounds in the Copolymerization of Ethylene/.alpha.-Olefins," in Macromol. Symp. 97:109-118 (1995), provides a similar system for preparing a polymer composition having a bimodal composition using a two-component catalyst comprised of a metallocene (Cp.sub.2 ZrCl.sub.2) and either Ph.sub.3 C.sup.+ !B(C.sub.6 F.sub.5).sub.4.sup.- ! or PhMe.sub.2 NH.sup.+ !B(C.sub.6 F.sub.5).sub.4.sup.- !.
PCT Publication No. WO92/00333, inventors Canich et al., and EP 416,815 A2, inventors Stevens et al., are also of interest insofar as the references describe metallocene catalysts for preparing polyolefins. Canich et al. describes metallocene catalyst compositions for producing high molecular weight polyolefins having a relatively narrow molecular weight distribution, wherein the catalyst composition is comprised of (1) a metallocene containing a Group IVB transition metal coordinated to a cyclopentadienyl ligand, and (2) a coordination complex such as an anionic complex containing a plurality of boron atoms, which serves as a catalyst activator. The metallocene catalysts described may be mononuclear or binuclear (i.e., containing one or two metal atoms which serve as the active sites); the binuclear compounds dissociate during polymerization. Stevens et al. also pertains to metallocene catalysts to prepare addition polymers, particularly homopolymers and copolymers of olefins, diolefins, "hindered" aliphatic vinyl monomers and vinylidene aromatic monomers. The Stevens et al. catalysts are metal coordination complexes having constrained geometry, and are used in conjunction with a cocatalyst compound or composition to form a complete catalytic system. The constrained geometry of the catalysts is stated to be of key importance insofar as the metal atom in the metallocene presumably is a more "exposed" active site.
Thus, the art provides metallocene catalyst compositions for producing polymers, particular polyolefins, which have a bimodal or multimodal molecular weight distribution. However, prior catalysts and catalyst compositions, as described above, either require two or more components, e.g., two catalysts used in combination, or involve binuclear compounds which break apart into two separate components during the polymerization process (as in the bimetallic catalyst disclosed by Canich et al.), giving rise to potential manufacturing problems, e.g., phase separation or the like, and/or loss of control over the molecular weight distribution of the polymer composition prepared. In addition, the known metallocene catalysts can be relatively difficult and time-consuming to synthesize, requiring expensive equipment, extreme reaction conditions, and multi-step processes which ultimately result in a low yield of the desired product.
Accordingly, there is a need in the art for a simpler polymerization catalyst which does not require a second catalyst, retains its structure during the polymerization process, and is relatively simple to synthesize. The novel metallocene compounds of the invention address the aforementioned need in the art and represent a significant advance in the field of catalysis. The compounds are binuclear or multinuclear metallocenes, preferably although not necessarily containing two or more distinct and chemically different active sites, and can be used in a variety of contexts. A preferred use is in the production of polymer compositions that are bimodal or multimodal in nature, typically, although not necessarily, having a desired bimodal or multimodal weight distribution. The catalysts allow for a high degree of control over both the compositional distribution and molecular weight distribution of the final polymer composition, and provide for all of the advantages typically associated with metallocene catalysts, i.e., versatility and use in conjunction with a variety of monomer types, the ability to control the degree of vinyl unsaturation in the polymeric product, the capability of providing isotactic or syndiotactic polymers, and the like. In addition to their utility as polymerization catalysts, the novel metallocenes are also useful in catalyzing hydrogenation. The novel compounds may be supported or used as homogeneous catalysts.